1. Department of Biological
Sciences and Institute of Structural and Molecular Biology, Birkbeck
College, University of London, Malet Street, London WC1E 7HX, U.K.
2. I07, Diamond Light Source, Diamond House, Harwell Science & Innovation Campus, Didcot, Oxfordshire OX11 0DE, U.K.
3. Institut Laue-Langevin, 71 avenue des Martyrs - CS 20156-38042, Grenoble 38000 Cedex 9, France